Various publications, including patents, published applications, technical articles and scholarly articles are cited throughout the specification. Each of these cited publications is incorporated by reference herein, in its entirety.
Degenerative neurological diseases and other disorders of the central and peripheral nervous system are among the most debilitating that can be suffered by an individual, not only because of their physical effects, but also because of their permanence. In the past, a patient suffering from a neurodegenerative condition of the central or peripheral nervous system, such as Parkinson's disease, Alzheimer's disease or multiple sclerosis, to name a few, held little hope for recovery or cure.
Parkinson's disease (PD) is a common neurodegenerative disorder with no known cure. The degenerative process of PD causes a preferential loss of dopamine neurons within the substantia nigra pars compacta (SNc). In fact, the substantia nigra is the principal site of pathology in Parkinson's disease. Pigmented neurons of the substantia nigra project widely and diffusely to the caudateputamen (corpus striatum) and are specialized to synthesize and release dopamine. Symptoms of parkinsonism emerge when 75-80% of the dopaminergic innervation is destroyed, leading to motoric dysfunction which presents itself as slowness of movement, rigidity, rest tremor, and postural instability (Dawson T M et al. (2002) Nat. Neurosci. 5 Suppl:1058-1061).
Thus, at the cellular level, PD is characterized by a severe loss of dopamine (DA) neurons in the substantia nigra, a key structure in regulating the complex basal ganglia circuitry involved in producing motor behavior. In humans, due to the combined degeneration of the SNc and of striatum, antiparkinsonian therapy based on levadopa substitution eventually fails in more than 90% of patients (Wenning et al. (1999) J. Nueral. Transm. Suppl. 55:103-113). Current treatment strategies are based on maintenance therapy as no cure is currently available for reversing the neurodegenerative deficit observed in Parkinson's patients. Dopamine replacement therapy has met with some success in Parkinson's patients. Unfortunately, the efficacy of dopamine replacement therapy decreases progressively with continued degeneration of the nigrostriatal dopaminergic pathway.
Neurological damage and neurodegenerative diseases were long thought to be irreversible because of the inability of neurons and other cells of the nervous system to grow in the adult body. However, the recent advent of stem cell-based therapy for tissue repair and regeneration provides promising treatments for a number of neurodegenerative pathologies and other neurological disorders. Stem cells are capable of self-renewal and differentiation to generate a variety of mature neural cell lineages. Transplantation of such cells can be utilized as a clinical tool for reconstituting a target tissue, thereby restoring physiologic and anatomic functionality. The application of stem cell technology is wide-ranging, including tissue engineering, gene therapy delivery, and cell therapeutics, i.e., delivery of biotherapeutic agents to a target location via exogenously supplied living cells or cellular components that produce or contain those agents (For a review, see Tresco, P. A. et al., (2000) Advanced Drug Delivery Reviews 42:2-37). The identification of stem cells has stimulated research aimed at the selective generation of specific cell types for regenerative medicine.
Cell transplantation to replace lost neurons is a new approach for the treatment of progressive neurodegenerative diseases such as PD. One obstacle to realization of the therapeutic potential of stem cell technology has been the difficulty of obtaining sufficient numbers of stem cells. Embryonic, or fetal tissue, is one source of stem cells. Embryonic stem and progenitor cells have been isolated from a number of mammalian species, including humans, and several such cell types have been shown capable of self-renewal and expansion, as well differentiation into all neurological cell lineages (Svendsen, C. V. et al. (1997) Exp. Neurol. 148:135-146; Freed, C. R. et al. (2001) New Engl J. Med. 344(10):719; Burnstein, R. M. et al. (2003) Int. J. Biochem. Cell Biol. 36:702-713; Zhang, S-C. et al. (2001) Nat. Biotechnol. 19:1129-1133; Reubinoff, B. E. et al. (2001) Nat. Biotechnol. 19:1134-1140; Björklund, L. M. et al. (2002) Proc. Natl. Acad. Sci. USA 99(4):2344-2349). But the derivation of stem cells from embryonic or fetal sources has raised many ethical and moral issues that are desirable to avoid by identifying other sources of multipotent or pluripotent cells.
Stem cells with neural potency also have been isolated from adult tissues. Neural stem cells exist in the developing brain and in the adult nervous system. These cells can undergo expansion and can differentiate into neurons, astrocytes and oligodendrocytes. However, adult neural stem cells are rare, as well as being obtainable only by invasive procedures, and may have a more limited ability to expand in culture than do embryonic stem cells.
Other adult tissue may also yield progenitor cells useful for cell-based neural therapy. For instance, it has been reported recently that adult stem cells derived from bone marrow and skin can be expanded in culture and give rise to multiple lineages, including some neural lineages (Azizi, S. A. et al. (1998) Proc. Natl. Acad. Sci. USA 95:3908-3913; Li, Y. et al. (2001) Neurosci. Lett. 315:67-70). Intrastriatal and intranigral grafting of other cell types, such as neurons derived from human teratocarcinoma, and human umbilical cord blood mononuclear cells have also been tested in animal models of Parkinson's disease (Baker, K. A. et al. (2000) Exp. Neurol. 162:350-360; Ende, N. and R. Chen (2002) J. Med. 33(1-4):173-180).
Postpartum tissues, such as the umbilical cord and placenta, have generated interest as an alternative source of stem cells. For example, methods for recovery of stem cells by perfusion of the placenta or collection from umbilical cord blood or tissue have been described. A limitation of stem cell procurement from these methods has been an inadequate volume of cord blood or quantity of cells obtained, as well as heterogeneity in, or lack of characterization of, the populations of cells obtained from those sources.
Although protocols have been developed for the directed differentiation of stem cells into therapeutically relevant cell types, such as dopaminergic (DA) neurons for the treatment of Parkinson's (Isacson, O. et al. (1996) Neurosci. 75:827-837; Kim, J-H. et al. (2002) Nature 418:50-56; Barbieri, T. et al. (2003) Nat. Biotechnol. 21(10):1200-1207), the efficient generation of substantial numbers of DA neurons has not yet been reported. The ability to generate unlimited numbers of DA neurons that express the full complement of midbrain DA neuron markers would provide an important contribution to a cure for Parkinson's disease.
Thus, alternative sources of adequate supplies of cells having the ability to differentiate into an array of neural cell lineages remain in great demand. Moreover, no satisfactory method exists to repair the damage caused by neuropathies such as Parkinson's disease (Parkinsonism). A reliable, well-characterized and plentiful supply of substantially homogeneous populations of such cells having the ability to differentiate into an array of neural lineages would be an advantage in a variety of diagnostic and therapeutic applications for neural repair, regeneration, and improvement, as well as applications for improvements in behavior and neurological function, particularly in PD patients.